The present invention relates generally to fuel delivery systems for engines, especially gas turbine engines, and more particularly to such fuel delivery systems for aircraft such as helicopters incorporating engine torque and temperature limiting features. The present invention provides an anticipatory correction of fuel flow to minimize problems in helicopter performance such as rotor droop.
Helicopter rotor blades are typically variable pitch and frequently that pitch is automatically varied during each revolution to compensate for differences in lift between the blade moving in the direction of helicopter motion and the blade moving in the opposite direction. Collective pitch is a measure of the average blade pitch and that average blade pitch is controllable by the pilot.
When a helicopter pilot causes an increase in rotor blade pitch, the blade angle of attack is increased (the blade takes a bigger bite of air) and the engine slows due to the increased load. A conventional engine governor will sense this decrease in speed and increase the fuel flow in an attempt to resume the previous speed. This temporary dip in rotor speed is called rotor droop. The opposite result occurs with a decreased pitch command.
Many current control systems for turbine engines provide limited anticipation to collective pulls which allows the rotor to droop down to 95%. In addition, these systems do not provide temperature limiting (either during engine start or in flight) or torque limiting during flight or acceleration contouring. The pilot has to control the helicopter to prevent exceedences on temperature as well as torque. This adds a tremendous burden to the pilot.
An audible alarm indicative of over-stress limits including temperature, output torque and engine speed, instructing the pilot to provide the corrective action is shown in U.S. Patent 4,619,110. A first low level signal is initiated when the operating parameter in question reaches a predetermined level below the allowable limit, and the audio signal is increased when the allowable limit is approached. The system automatically limits fuel flow to avoid over-stressing the engine, but this limiting may be over-ridden under emergency conditions. Fuel flow limiting is accomplished by bleeding some air from a pneumatic fuel controller. Water and/or additional fuel may also be automatically injected into the engine fuel flow. This patented scheme utilizes pneumatic control of one fuel valve for all fuel flow control. Flowing pneumatic systems are limited in terms of reliability because of the inherent issues associated with flowing air laden with the products of combustion through computational control circuits. Another limitation of such control systems is the failure to offer redundancy regarding power turbine governing. Subsequently system safety could be improved in this area.
New full authority digital electronic control (FADEC) systems that have been developed in recent years offer solutions to these problems but these systems are high cost and are only offered on new expensive model helicopters.
In our aforementioned copending application, an engine overtemperature avoidance technique operable only during engine start-up monitors engine temperature and diminishes fuel flow to the engine when that monitored temperature exceeds a threshold temperature. While this system admirably performs its intended function of avoiding thermal stress to the engine during start-up, its function ceases upon the engine reaching normal idle speed.
It is desirable to provide automatic power turbine governing and torque limiting in a continuous, economical, relatively simple, redundant and retrofitable way. It is also desirable to anticipate engine requirements and to modify the fuel supply rate as required to minimize changes in engine speed due to changes in engine load.
The present invention provides solutions to the above problems by providing a fuel burning engine fail-safe stress avoidance system which monitors a number of engine operating parameters and inhibits the flow of fuel to the engine when one or more of the parameters exceeds its prescribed limit. The avoidance system assumes control from a pneumatic engine governor and relinquishes that control upon detecting an avoidance system malfunction. The system offers most of the advantages of a FADEC system at a fraction of the cost. It is affordable to most users both as an upgrade and to new original equipment manufacturers.
In general, when a engine reaches normal idle speed, the function of a pneumatic engine speed governor is transferred to a controllable fuel flow bypass which selectively diverts fuel exiting a fuel flow control valve away from the engine and back to a fuel pump thereby decreasing the rate of fuel flow to the engine. The fuel bypass includes a normally closed solenoid operable valve and a torque motor control valve connected in series between the control valve outlet and the fuel pump inlet. The pneumatic governor is disabled by energizing a normally open solenoid valve connecting an air supply source to the pneumatic governor. This fuel bypass is operable independently from the conventional bypass which diverts fuel exiting the fuel control valve away from the engine and back to the fuel pump to maintain a relatively constant pressure differential across the fuel control valve and diverts fuel exiting the control valve away from the engine and back to the fuel pump to maintain at least one of turbine speed, gas generator speed, engine output torque and engine temperature below respective threshold levels.